Mr. Krabs grew up in a poor home, with few possessions and wearing patched up clothing. This may be what led to his fascination/obsession with money. However, continuity in the Spongebob universe is flimsy at best, and another episode states Mr. Krabs began the Krusty Krab after an unnamed war.
Mr. Krabs' military service has been hinted at in other episodes, and its worth noting none of his old navy friends suggested that he had ever been cheap or money driven. So possibly his unhealthy fixation was latent during his younger years.
Mr. Krabs lives for the two thing: money and more money. He will literally do anything thing to either get money or save money.
He once forced an expired Krabby Patty that SpongeBob had found under the grill, on every customer because he thought throwing it out was a waste of money.
His obsession with money borders on insanity. He once saw SpongeBob pick up a penny off the sidewalk and tried everything thing from opening a fake movie theater, to throwing a fake fund raiser to get it from him. Finally he broke into SpongeBob's house to get the penny, when SpongeBob found him he said it wasn't a penny but a piece of gum for his dried gum collection. Appearing to let go of his penny obsession, left Spongebob's house only to dig up his yard believing SpongeBob buried the penny.